ES2551005T3 - Manufacturing process of martensitic steel of very high elastic limit and sheet or piece obtained in this way - Google Patents

Abstract

Manufacturing process of a steel sheet of less than 3 millimeters thick, with a totally martensitic structure, with an elasticity limit of more than 1300 MPa, which includes the successive stages and in this order according to which: - a semi-product of steel whose composition comprises, the contents being expressed by weight, 0.15% <= C <= 0.40% 1.5% <= Mn <= 3% 0.005% <= Si <= 2% 0.005% <= Al <= 0.1% S <= 0.05% P <= 0.1% 0.025% <= Nb <= 0.1% and optionally: 0.01% <= Ti <= 0.1% 0% < = Cr <= 4% 0% <= Mo <= 2% 0.0005% <= B <= 0.005% 0.0005% <= Ca <= 0.005% being the rest of the iron composition and unavoidable impurities resulting from processing, - the semi-product is reheated to a temperature T1 between 1050 ° C and 1250 ° C, then - a roughing laminate of said superheated semi-product is carried out, at a temperature T2 between 1050 and 1150 ° C , with an accumulated reduction rate ε a greater than 1 00% so that a sheet with an austenitic structure not fully recrystallized with an average grain size of less than 40 micrometers is obtained, then - said sheet is not completely cooled to a temperature T3 between 970 ° C and Ar3 + 30 ° C, at a speed VR1 of more than 2 ° C / s, then - a hot rolling of finishing at said temperature T3 is carried out, of said sheet not completely refrigerated, with a cumulative reduction rate εb greater than 50% so that a sheet, of a thickness of less than 3 millimeters, then - said sheet is cooled at a VR2 speed greater than the critical speed of martensitic quenching.

Description

E12724659

10-22-2015

DESCRIPTION

Manufacturing process of martensitic steel of very high elastic limit and sheet or piece obtained in this way

[0001] The invention relates to a process of steel sheets of less than 3 millimeters thick with a fully martensitic structure with a mechanical resistance greater than that which could be obtained by a simple rapid cooling treatment with martensitic tempering and resistance properties mechanical and elongation that allow its application to the manufacture of energy absorption parts in vehicles

10 cars

[0002] In certain applications, it is sought to make parts from steel sheet with very high mechanical resistance. This type of combination is particularly desirable in the automotive industry where significant lightening of vehicles is sought. This can be obtained especially thanks to the use of

15 pieces of steels with very high mechanical characteristics whose microstructure is totally martensitic. Anti-intrusion parts, of structure or that participate in the safety of motor vehicles such as: bumper crossbars, door or upright reinforcements, wheel arm, which need, for example, such characteristics. Its thickness is preferably less than 3 millimeters.

20 [0003] It is sought to obtain sheets with an even greater mechanical resistance. The possibility of increasing the mechanical strength of a steel with martensitic structure by means of an addition of carbon is well known. However, this higher carbon content decreases the welding capacity of the sheets or of the parts manufactured from these sheets and increases the risk of cracking together with the presence of hydrogen.

25 [0004] It is therefore sought to have a method of manufacturing steel sheets that does not have the above drawbacks, which would be provided with a breaking strength of more than 50 MPa which could be obtained thanks to austenitization followed of a simple martensitic tempering of the steel in question. The inventors have shown that, for carbon contents ranging from 0.15 to 0.40% by weight, the

30 tensile tensile strength Rm of plates, of steels manufactured by total austenitization followed by a simple martensitic hardening, only depended practically on the carbon content and was attached to it with very good precision, according to the expression (1): Rm (megapascals) = 3220 (C) + 908.

[0005] In this expression, (C) designates the carbon content of the steel expressed in weight percentage.

35 With a given C carbon content of a steel, a manufacturing process is therefore sought that allows to obtain a breaking strength greater than 50 MPa at the expression (1), that is to say a resistance greater than 3220 (C) + 958 MPa for this steel. It is sought to have a procedure that allows the manufacture of very high elastic limit sheet, that is to say greater than 1300 MPa. It also seeks to have a procedure that allows the manufacture of directly usable plates, that is, without the need for a treatment of

40 tempered after tempering.

[0006] These sheets must be weldable by the usual procedures and not consist of expensive additions of alloy elements.

[0007] The present invention aims to solve the problems mentioned above. It has in particular the objective of making plates of thickness less than 3 millimeters available with an elasticity limit greater than 1300 MPa, a mechanical tensile strength, expressed in megapascals, greater than (3220 (C) +958) MPa and , preferably, a total elongation greater than 3%.

[0008] In this objective, the invention has as its object a manufacturing process of a thickness less than 3 millimeters of totally martensitic structure with an elasticity limit greater than 1300 MPa, which comprises the successive stages and in this order according to which:

- a semi-product of steel is supplied whose composition comprises, the contents being expressed in

55 weight: 0.15% ≤ C ≤ 0.40%, 1.5% ≤ Mn ≤ 3%, 0.005% ≤ If ≤ 2%, 0.005% ≤ Al ≤ 0.1%, S ≤ 0.05%, P≤ 0.1%, 0.025% ≤ Nb≤0.1% and optionally: 0.01% ≤ Ti≤0.1%, 0% ≤ Cr≤ 4%, 0% ≤ Mo ≤2%, 0.0005 % ≤ B ≤ 0.005%, 0.0005% ≤ Ca ≤ 0.005%, with the rest of the iron composition and the inevitable impurities resulting from processing.

E12724659

10-22-2015

-the semi-product is reheated to a temperature T1 between 1050 ° C and 1250 ° C, then a roughing laminate of the reheated semi-product is made, at a temperature T2 between 1050 and 1150 ° C, with a cumulative reduction rate εa greater than 100% so that a sheet with an austenitic structure not fully recrystallized of average grain size less than 40 micrometers is obtained, then

5-the sheet is not completely cooled to a temperature T3 between 970 ° C and Ar3 + 30 ° C, so as to avoid a transformation of the austenite, at a speed VR1 greater than 2 ° C / s, then

- a hot rolling of finishing at temperature T3 is carried out, of the sheet not completely refrigerated, with

10 an accumulated reduction index εb greater than 50% so that a sheet is obtained, less than 3 millimeters thick, then the sheet is cooled at a VR2 speed greater than the critical speed of martensitic tempering.

[0009] According to a preferred mode, the average size of austenitic grains is less than 5 micrometers.

[0010] Preferably, the sheet is subjected to a further heat treatment of tempering at a temperature T4 between 150 and 600 ° C for a duration between 5 and 30 minutes.

[0011] The invention also has as its object a steel sheet of thickness less than 3 millimeters not

20 yield limit of elasticity greater than 1300 MPa, obtained by a method according to one of the previous manufacturing modes, of totally martensitic structure, which has an average slat size of less than 1.2 micrometers, the average elongation factor being included of the slats between 2 and 5.

[0012] The object of the invention is even a steel sheet of less than 3 millimeters thick obtained by the

25 procedure with previous tempering treatment, the steel having a totally martensitic structure with an average slat size of less than 1.2 micrometers, the average elongation factor of the slats being comprised between 2 and 5.

[0013] The composition of the steels applied in the process according to the invention will now be detailed:

30 When the carbon content of the steel is less than 0.15% by weight, the hardenability of the steel is insufficient and it is not possible to obtain a fully martensitic structure taking into account the procedure applied. When this content is greater than 0.40%, welded joints made from these sheets or these pieces have insufficient toughness. The optimum carbon content for the application of the invention is

35 between 0.16 and 0.28%.

[0014] Manganese lowers the starting temperature of martensite formation and slows down the decomposition of austenite. In order to obtain sufficient effects, the manganese content should not be less than 1.5%. On the other hand, when the manganese content exceeds 3%, segregated areas are presented in quantity

Excessive what affects the application of the invention. A preferred range for the application of the invention is 1.8 to 2.5% Mn.

[0015] The silicon content must be greater than 0.005% so that it participates in the deoxidation of the liquid phase steel. Silicon should not exceed 2% by weight due to the formation of surface oxides that significantly reduce the coating, in the case where it would be desirable to cover the sheet by passage in a metallic coating bath, especially by continuous galvanization.

[0016] The aluminum content of the steel according to the invention is not less than 0.005% so that a sufficient deoxidation of the steel in the liquid state is obtained. When the aluminum content is greater than 0.1% by 50 weight, casting problems may occur. Alumina inclusions may also be formed in too large an amount or size that play a damaging role on toughness.

[0017] The sulfur and phosphorus contents of the steel are respectively limited to 0.05 and 0.1% to avoid a reduction in ductility or toughness of the pieces or of the sheets manufactured according to the invention.

[0018] The steel also contains niobium in an amount between 0.025 and 0.1% and optionally titanium in an amount between 0.01 and 0.1%. These additions of niobium and possibly titanium allow the application of the process according to the invention by delaying the recrystallization of austenite at high temperature and allowing obtaining a sufficiently fine grain size at high temperature.

E12724659

10-22-2015

[0019] Chromium and molybdenum are very effective elements to delay the transformation of austenite and can optionally be used for the application of the invention. These elements have the effect of separating the domains of ferrite-perlitic and bainitic transformation, intervening the ferrite-perlitic transformation in some

5 temperatures higher than the bainitic transformation. These transformation domains are then presented in the form of two quite different "noses" in an isothermal transformation diagram (Temperature-Time Transformation).

[0020] The chromium content must be less than or equal to 4%. Beyond this content, its effect on the

10 hardenability is almost saturated; a supplementary addition is then expensive without corresponding beneficial effect.

[0021] The molybdenum content must not exceed 2%, however, due to its excessive cost.

[0022] On an optional basis, steel may also contain boron: in fact, the significant deformation of austenite can accelerate the transformation into ferrite with cooling, a phenomenon that should be avoided. An addition of boron, in an amount between 0.0005 and 0.005% by weight, can be protected from an early ferritic transformation.

[0023] On an optional basis, steel may also contain calcium in an amount between 0.0005 and 0.005%: in combination with oxygen and sulfur, calcium makes it possible to prevent the formation of large inclusions that are detrimental to the ductility of the sheets or of the parts manufactured in this way.

[0024] The rest of the steel composition consists of iron and unavoidable impurities that result from processing.

[0025] The steel sheets manufactured according to the invention are characterized by a totally martensitic structure in slats of great fineness: due to the specific thermomechanical cycle and composition, the average size of the martensitic slats is less than 1.2 micrometers and its average elongation factor is between 2 and 5. These microstructural characteristics are determined, for example, by observing the microstructure by Scanning Electron Microscopy by means of a cannon with field effect ("MEB-FEG" technique) with a higher magnification. at 1200x, coupled to an EBSD detector ("Electron Backscatter Diffraction"). It is defined that two adjacent slats are different when their disorientation is greater than 5 degrees. The average slat size is defined by the method of interceptions known per se: the average size of the 35 slats intercepted by randomly defined lines with respect to the microstructure is evaluated. The measurement is carried out on at least 1,000 martensitic slats so that a representative average value is obtained. The morphology of the individualized slats is then determined by image analysis by means of programs known in themselves: the maximum dimension lmax and minimum lmin of each martensitic strip is determined

and its elongation factor

image 1 In order to be statistically representative, this observation is about at least image2

40 1,000 martensitic slats. The average elongation factor

It is determined below for the set of these slats observed.

[0026] The process of manufacturing hot rolled sheets according to the invention consists of the following steps:

45 A steel semi-product whose composition has been discussed above is supplied first. This semi-product can be presented, for example, in the form of a section resulting from continuous casting, thin section or ingot. As an indicative example, a continuous casting section has a thickness of the order of 200 mm, a thin section, a thickness of the order of 50-80 mm.

50 This semi-product is reheated to a temperature T1 between 1050 ° C and 1250 ° C. The temperature T1 is higher than Ac3, total transformation temperature in austenite upon heating. This overheating therefore allows to obtain a complete austenitization of the steel as well as the dissolution of possible niobium carbonitrides that exist in the semi-product. This overheating stage also allows the different

55 subsequent hot rolling operations to be presented: a laminate called

E12724659

10-22-2015

roughing of the semi-product: this roughing laminate is carried out at a temperature T2 between 1050 and 1150 ° C. The cumulative reduction rate of the different stages of roughing rolling is indicated as εa. If eia designates the thickness of the semi-product before the hot rolling of roughing and image3 fa the thickness of the sheet after

this laminate, the accumulated reduction index is defined by According to the invention, the index of

The reduction εa must be greater than 100%, that is to say greater than 1. Under these rolling conditions, the presence of niobium, and optionally titanium, delays recrystallization and allows obtaining an austenite not fully recrystallized at high temperature. The average size of austenitic grain obtained in this way is less than 40 micrometers, even at 5 micrometers when the niobium content is between 0.030 and 0.050%. This grain size can be measured for example thanks to some tests in which it is tempered directly after

10 laminated the sheet. A polished and attacked section of it is observed below, the attack being carried out thanks to a reagent known in itself, such as for example the Béchet-Beaujard reagent that reveals the old joints of austenitic grains.

It is then cooled not completely, that is to an intermediate temperature T3, the sheet at a

15 VR1 speed exceeding 2 ºC / s, so that a transformation and eventual recrystallization of the austenite is avoided, then a hot rolling of the sheet finish with an accumulated reduction index εb greater than 50% is carried out. If e12 designates the thickness of the sheet before finishing laminate and image4 f2 the thickness of

the sheet after this laminate, the accumulated reduction index is defined by

This finishing laminate is made at a temperature T3 between 970 and Ar3 + 30 ° C, designating Ar3 as the temperature of

20 start of transformation of austenite to refrigeration. This allows an austenite deformed with fine grains to be obtained at the end of the finishing laminate, not having this tendency to recrystallize. This sheet is then cooled to a VR2 speed higher than the martensitic critical tempering speed and thus a sheet characterized by a very fine martensitic structure is obtained whose mechanical properties are superior to those that can be obtained by a simple heat treatment of tempering.

[0027] Although the above procedure describes the manufacture of sheets, that is to say flat products, from sections, the invention is not limited to this geometry and this type of products and may also be adapted to the manufacture of long products, of bars, profiled, by successive stages of hot deformation.

[0028] Steel sheets can be used as is or subjected to a tempering heat treatment carried out at a temperature T4 between 150 and 600 ° C for a duration between 5 and 30 minutes. This tempering treatment generally has the effect of increasing the ductility at the cost of a decrease in the elasticity and strength limit. The inventors have shown however that the process according to

The invention, which confers a mechanical tensile strength of at least 50 MPa higher than that obtained after conventional tempering, retained this advantage even after a tempering treatment with temperatures ranging from 150 to 600 ° C. The fineness characteristics of the microstructure are preserved by this tempering treatment.

[0029] By way of non-limiting example, the following results will show the advantageous characteristics conferred by the invention.

Example:

[0030] Steel semi-products have been supplied whose compositions, expressed in weight contents (%) are as follows:

C

Mn Yes Cr Mo To the S P Nb You B AC

TO

0.27 1.91 0.01 0.01 0.01 0.03 0.003 0.020 0.042 0.010 0.0016 0.001

B

0.198 1.94 0.01 1,909 0.01 0.03 0.003 0.020 0.003 0.012 0.0014 0.0004

The underlined values are not in accordance with the invention.

[0031] Some 31 mm thick semi-products have been reheated and kept 30 minutes at a temperature

50 T1 of 1250 ° C, then subjected to a rolling in 4 passes at a temperature T2 of 1100 ° C with a cumulative reduction rate ε1 of 164%, that is to say a thickness of 6 mm. In this state, at high temperature after roughing, the structure is completely austenitic, not completely recrystallized with an average size of

E12724659

10-22-2015

30 micrometer grain. The sheets obtained in this way have then been cooled at a rate of 3 ° C / s to a temperature T3 between 955 ° C and 840 ° C, the latter being equal to Ar3 + 60 ° C. The sheets have been laminated in this temperature range in 5 passes with a cumulative reduction rate εb of 76%, that is to a thickness of 2.8 mm, then cooled to room temperature

5 with a speed of 80 ° C / s so that a completely martensitic microstructure is obtained.

[0032] By comparison, steel sheets of the above composition have been heated to a temperature of 1250 ° C, kept 30 minutes at this temperature, then cooled to water so that a completely martensitic microstructure is obtained (reference condition)

[0033] By means of tensile tests, the elasticity limit Re, the breaking strength Rm and the total elongation A of the plates obtained by these different manufacturing modes have been determined. The estimated value of the resistance after simple martensitic hardening (3220 (C) +908 (MPa), as well as the difference ΔRm between this estimated value and the effectively measured resistance has also been included.

fifteen

Steel

Proof Reduction temperature T3 (ºC) Re (MPa) Rm (MPa) TO (%) 3220 (C) +908 (MPa) ΔRm (MPa)

TO

A1 955 1410 1840 5.2 1777 63

A2

860 1584 1949 4.9 1777 172

B

B1 840 1270 1692 6.5 1545 147

B2

Without 1223 1576 6.9 1545 31

Test conditions and mechanical results obtained Underlined values: not in accordance with the invention

[0034] Steel B does not contain enough niobium: an elasticity limit of 1300 MPa is not reached, both after simple martensitic hardening (test B2) and in the case of a laminate with roughing and finishing at temperature T3 (test B1)

[0035] In the case of the B2 test (simple martensitic quenching), it is observed that the estimated resistance value (1545 MPa) from the expression (1) is close to that determined experimentally (1576 MPa)

[0036] The microstructure of the plates obtained by Scanning Electron Microscopy was also observed by means of a cannon with field effect ("MEB-FEG" technique) and EBSD detector and quantified the

average size of the slats of the martensitic structure as well as its average elongation factor

image5

[0037] In tests A1 and A2, the process according to the invention allows obtaining a martensitic structure with an average slat size of 0.9 micrometers and an elongation factor of 3. This structure is clearly

30 finer than that observed after simple martensitic quenching, whose average slat size is of the order of 2 micrometers.

[0038] In tests A1 and A2 according to the invention, the values of ΔRm are respectively 63 and 172 MPa respectively. The process according to the invention thus allows to obtain significantly higher mechanical resistance values than would be obtained by a simple martensitic tempering. In the case of the A2 test, for example, this increase in resistance (172 MPa) is equivalent to what would be obtained, according to the ratio (1), thanks to a simple martensitic hardening applied to steels in which a supplementary addition of 0 , Approximately 05% would have been made. Such an increase in carbon content would nevertheless have detrimental consequences with regard to weldability and toughness, while the procedure according to

The invention allows to increase the mechanical resistance without these inconveniences.

[0039] The sheets manufactured according to the invention, because of their lower carbon content, have a good welding capacity by the usual procedures, in particular spot resistance welding. They also have a good ability to be coated, for example by

45 galvanization or aluminization to continuous tempering.

[0040] Thus, the invention allows the manufacture of sheets of thickness less than 3 millimeters or bare or coated with very high mechanical characteristics, in very satisfactory economic conditions.

Claims (4)

1. Procedure for manufacturing a sheet steel of less than 3 millimeters thick, of structure totally martensitic, with an elasticity limit greater than 1300 MPa, which includes the successive stages and in 5 this order according to which: - a semi-product of steel is supplied whose composition comprises, the contents being expressed by weight, 10 0.15% ≤ C ≤ 0.40% 1.5% ≤ Mn ≤ 3% 0.005% ≤ Yes ≤ 2% 0.005% ≤ Al ≤ 0.1% S ≤ 0.05% 15 P ≤ 0.1% 0.025% ≤ Nb ≤ 0.1% and optionally: 0.01% ≤ Ti ≤ 0.1% 0% ≤ Cr ≤ 4% 20 0% ≤ Mo ≤ 2% 0.0005% ≤ B ≤ 0.005% 0.0005% ≤ Ca ≤ 0.005% the rest of the iron composition and the inevitable impurities resulting from the elaboration being constituted, 25 -the semi-product is reheated to a temperature T1 between 1050 ° C and 1250 ° C, then a roughing laminate of said superheated semi-product is made, at a temperature T2 between 1050 and 1150 ° C, with an index of accumulated reduction εa greater than 100% so that a sheet with an austenitic structure not fully recrystallized of average grain size less than 40 micrometers is obtained, then 30-said sheet is not completely cooled to a temperature T3 between 970 ° C and Ar3 + 30 ° C, at a speed VR1 greater than 2 ° C / s, then -a finished hot rolling at said temperature T3, of said sheet not completely refrigerated, with a cumulative reduction index εb greater than 50% so that a sheet is obtained, less than 3 millimeters thick, then 35-said sheet is cooled at a VR2 speed greater than the critical speed of martensitic quenching. 2. Method of manufacturing a steel sheet according to claim 1, characterized in that said average size of austenitic grain is less than 5 micrometers. Method of manufacturing a steel sheet according to any of claims 1 or 2, characterized in that said sheet is subjected to a subsequent heat treatment of tempering at a temperature T4, between 150 and 600 ° C for a duration between 5 and 30 minutes 4. Steel sheet with a thickness of less than 3 millimeters, with an elastic limit of more than 1300 MPa, 45 obtained by a method according to any one of claims 1 or 2, of totally martensitic structure, having an average slat size of less than 1.2 micrometers, the average elongation factor of said slats being comprised between 2 and 5. 5. Steel sheet obtained by a method according to claim 3, of totally structure 50 martensitic, which has an average slat size of less than 1.2 micrometers, the average elongation factor of said slats being between 2 and 5. 7